1. Field of the Invention
The present invention relates to a projection optical system and an image projector including the projection optical system.
2. Description of the Related Art
Recently, liquid crystal projectors, which are widely known as image projectors, have incorporated liquid crystal panels of higher resolution, have been improved in brightness with higher efficiency of light source lamps, and have been reduced in price. Further, small-size, light-weight image projectors using a digital micro-mirror device (DMD) or the like have been popular, so that image projectors have been widely used not only in offices or at school but also at home. In particular, front-type projectors have been improved in portability to be used at small meetings of a few people.
Ultra short focus projectors have the advantage of letting a presenter make a presentation without the shadow of the presenter appearing on a projection surface, but have the disadvantage of not being usable at a distance from the projection surface and thus being required to be embedded in the projection surface if the projection distance is shorter than the overall length of the optical system.
Such a disadvantage is eliminated by folding back an optical path with a folding mirror.
In these projectors, if optical systems are exposed, there arises the problem of dust deposition or the like. Therefore, the conventional projectors are equipped with a dustproof cover that provides protection against dust. (See, for example, Japanese Laid-Open Patent Application No. 2010-152263, Japanese Laid-Open Patent Application No. 2010-152264, Japanese Patent No. 4396769, and Japanese Laid-Open Patent Application No. 2009-276789.)